Recently, in the field of medical care, diagnosis using a positron emission tomography (which will be abbreviated herein as “PET”) scanner has been carried out more and more often. Thus, to realize a PET scanner with even higher performance, searches for better scintillator materials have been conducted.
The scintillator materials for use to make a PET scanner need to detect a γ ray. To meet those needs, single crystal scintillator materials, including BGO (bismuth germanium oxide), LSO (lutetium silicon oxide), GSO (gadolinium silicon oxide) and LYSO (lutetium yttrium silicon oxide), have been used so far to make a PET scanner. The properties of a scintillator material are evaluated by its emission intensity (i.e., fluorescence output), fluorescence decay time, and energy resolution, for example. All of the single crystal materials mentioned above have properties that are good enough to use them to make a PET scanner. And as for a method of growing such a single crystal, a melt growth process such a Czochralski process or a Bridgman process has been used extensively on an industrial basis.
To make the PET more popular, however, the throughput of the diagnosis should be increased. But the throughput cannot be increased unless a single crystal scintillator material, of which the intensity of emission is greater than, but the fluorescence decay time is shorter than, conventional scintillator materials, is developed.
Patent Document No. 1 discloses GSO activated with a dopant Ce (cerium). On the other hand, Patent Documents Nos. 2 and 3 disclose cerium doped lutetium borate materials. Cerium doped lutetium borate has a high intensity of emission and a short fluorescence decay time, and therefore, is considered a promising scintillator material. Patent Document No. 3 also suggests that the cerium doped lutetium borate material be applied to the field of PET. However, the cerium doped lutetium borate material disclosed in that document is just powder. Thus, a single crystal of cerium doped lutetium borate that is big enough to use it in the PET cannot be formed by the method disclosed in Patent Document No. 2 or 3.
As for lutetium borate, its phase transition point (of about 1,350° C.) that involves a significant volumetric change is located in a lower temperature range than its melting point (of 1,650° C.). That is why according to a conventional single crystal growing process, in which the starting material should be heated to a temperature that is high enough to melt or dissolve the material, when the melt being cooled passes the phase transition point, its volume will expand so much that the crystal will collapse, which is a serious problem. Patent Document No. 4 discloses a method for producing a single crystal scintillator material by checking the phase transition of a crystalline material with element Sc, Ga or In added to a lutetium borate material. However, a lutetium borate based single crystal formed by the method disclosed in Patent Document No. 4 would also cause some deterioration such as a decrease in density or emission intensity due to the introduction of the additive element.
In order to overcome these problems, the applicant of the present application discloses a cerium doped lutetium borate single crystal formed by a flux method that uses a lead borate solvent in PCT/JP2008/1717 (filed on Jul. 1, 2008).